1. Field of the Invention:
This invention relates to a toroidal continuous variable transmission adapted to transmit the rotation of an input disc, which is provided so as to be opposed to an output disc, to the output disc via power rollers, which are adapted to be rotated with the power rollers contacting the input and output discs, by changing a speed of the mentioned rotation in a stageless manner in accordance with an angle of turn of the power rollers.
2. Description of the Prior Art:
A toroidal continuous variable transmission mounted on an automobile is generally a double cavity type transmission in which two variable speed change units are provided on the same shaft. This toroidal continuous variable transmission has an input shaft into which an engine output is inputted, a pair of input discs supported rotatably on the input shaft, output discs provided so as to be opposed to the input discs respectively, and supported rotatably on the input shaft, power rollers provided between the opposed input discs and output discs, adapted to transmit torque from the input discs to the output discs and capable of being turned, a member for unitarily connecting the opposed output discs together, and pressure means provided between a flange of the input shaft and input discs and adapted to work on the input discs and change a pressing force of the power rollers in accordance with the magnitude of the input torque, the rotation of the input discs being subjected to a stageless speed change in accordance with an angle of turn of the power rollers and transmitted to the output discs.
In this toroidal continuous variable transmission, the turning of the power rollers is done by a variable speed change unit. Various types of variable speed change units (Japanese Patent Laid-Open Nos. 82065/1986, 46060/1987 and 203957/1988) have heretofore been known. The known variable speed change units include, for example, a variable speed change unit shown in FIG. 6. FIG. 6 shows one variable speed change unit but, in the case of a double cavity type toroidal continuous variable transmission, a structure capable of supplying a hydraulic pressure to two variable speed change units is provided.
As shown in FIG. 6, a variable speed change unit 1 in a toroidal continuous variable transition 1 is provided with input discs 3 on an input shaft 3S, and output discs on an output shaft similarly, though they are not shown. A pair of power rollers 2 are provided in an opposed state so that they are held between the input discs 3 and output discs which are provided so as to be opposed to each other, and these power rollers 2 are supported rotatably on trunnions 4. The power rollers 2 are supported on the trunnions 4 via eccentric shafts 5. The trunnions 4 are supported pivotably and axially movably on a transmission casing (not shown). Namely, each trunnion 4 has a trunnion shaft 6, and can be turned therearound and moved in the direction of the axis thereof. Pistons 7 are fixed to the shaft 6 of the trunnions 4 so that they can be moved slidingly in hydraulic cylinders 8 formed in the transmission casing. The hydraulic cylinders 8 are provided therein with two cylinder chambers 8a, 8b defined by the pistons 7.
The cylinder chambers 8a, 8b in the hydraulic cylinders 8 communicate with a spool valve 10 via lines 9a, 9b. A spool 11 provided in the spool valve 10 is maintained in a neutral position by springs 12 provided at both ends thereof. The spool valve 10 is provided with an Sa port at one end thereof, and an Sb port at the other end thereof. A pilot pressure is supplied to the Sa port via a solenoid valve 13a, and to the Sb port via a solenoid valve 13b. The spool valve 10 also has a P communicating with a pump pressure (pressure source), an A port communicating with the cylinder chamber 8a via the lines 9a, a B port communicating with the cylinder chamber 8b via the conduit 9b, and T ports communicating with a drain to atmospheric pressure. The solenoid valves 13a, 13b are operated by a control signal outputted from a controller 14.
A precession cam 15 is connected to a free end of one trunnion shaft 6, and one end of a lever 16, an intermediate portion of which is pivotably connected, contacts the precession cam 15, the other end of the lever 16 being connected to a potentiometer 17. The potentiometer 17 is adapted to detect the axial displacement and an angle of turn of the shaft 6 of the trunnion 4 as an amount of composite displacement, and input a detected signal into the controller 14. This variable speed change unit is further provided with various kinds of sensors, such as a vehicle speed sensor 18, an engine revolution sensor 19 and a degree of opening of throttle sensor 20, and formed so that signals of speed change information including a vehicle speed, engine revolutions and a degree of opening of a throttle detected by these sensors are inputted into the controller 14.
The toroidal continuous variable transmission utilizes a principle that, when the trunnions 4 are displaced from neutral positions (positions in which the axes of rotation of the power rollers 2 cross those of rotation of the input discs 3 and output discs) in one of the directions of the trunnion shafts (put discs) in one of the directions of the trunnion shafts), the trunnions 4 are turned around the trunnion shafts 6 in a direction and at a speed which are in accordance with the speed and the amount of the mentioned displacement, a speed change being carried out by such turning movements of the trunnions 4.
The operation of this variable speed change unit will now be described. The controller 14 is adapted to compute an actual speed change ratio on the basis of an amount of composite displacement of the trunnions 4 detected by the potentiometer 17, set target displacement of the trunnions 4 in accordance with a difference between the actual and target speed change ratios, and output control signals to the solenoid valves 13a, 13b. Consequently, hydraulic pressures Sa, Sb are supplied from the solenoid valves 13a, 13b to both ends of the spool valve 10. When the relation between the hydraulic pressures Sa, Sb supplied to the spool valve 10 in this manner is Sa&gt;Sb, the spool 11 is shifted to left (FIG. 6), and the line 9a communicates with a power source P via the P port, the line 9b communicating with the drain via the T ports, a pressure Pa in the conduit 9a becoming higher than Pb in the line 9b (Pa&gt;Pb). As a result, due to a difference between the pressures in the cylinder chambers 8a, 8b, the left (FIG. 6) trunnion 4 is displaced downward, and the right trunnion 4 upward. In accordance with this displacement, the trunnions 4 are turned around the trunnion shafts 6 to start a speed change operation. A feedback control operation is carried out by the controller 14 so that the actual speed change ratio comes close to the target speed change ratio. As the actual speed change ratio comes close to the target speed ratio, the target displacement of the trunnions 4 approaches zero, and, when the actual speed change ratio has agreed with the target speed change ratio, the target displacement of the trunnions 4 becomes zero to finish the speed change operation.
According to this variable speed change unit, when an actual speed change ratio agrees with a target speed change ratio with the target displacement of the trunnions 4 at a zero level, the pressures applied to both ends of the spool valve 10 become equal (Sa=Sb9), and the spool 11 in the spool valve 10 returns to the neutral position, so that the pressures in the two cylinder chambers 8a, 8b become equal (Pa=Pb).
During the transmission of torque, the power rollers 2 receive from the input discs 3 and output discs a tangential force F in the directions of dual arrows F shown in FIG. 6. Accordingly, the trunnions 4 supporting the power rollers 2 receive a force F in the direction of the trunnion shafts the level of which is in accordance with transmission torque and speed change ratio, and is displaced finely in the direction of the axes of the trunnion shafts. These phenomena will now be described in detail with the attention paid to the right (FIG. 6) trunnion 4. When a force F in the direction of the trunnion shaft is applied to this trunnion 4, the relative piston 7 is displaced upward, and the pressure in the relative cylinder chamber 8b increases with that in the other cylinder chamber 8a decreasing, whereby a pressure difference occurs between the two cylinder chambers 8a, 8b. This pressure difference increases as the displacement amount of the piston 7 increases, and, when the pressure difference reaches a level which bears proportion to the level of the force F in the direction of the trunnion shaft, the piston 7 stops. Thus, during the transmission of torque, the trunnion 4 is finely displaced in the direction of the trunnion shaft due to the external force (arrow F) which the relative power roller 2 receives. As a result, the axial displacement of the trunnion shaft 6 unitarily provided on the trunnion 4 is detected by the potentiometer 17, and the controller 14 outputs control signals to the solenoid valves 13a, 13b so as to start a speed change operation again. Thus, when a conventional variable speed change unit receives such an external force F, a speed change operation is started again, though the speed change operation has finished. This prevents a speed change control operation from being carried out stably, and causes a decrease in the speed changing efficiency, and an increase in the heating value due to a number of repeated side slips.